Triad color screen having elements larger than mask apertures near screen center and smaller near periphery

ABSTRACT

A color television tube of the shadow mask type with aligned phosphor windows is provided with an increased light output by providing the phosphor window diameter for the central portion of the phosphor screen of the tube having a diameter greater than the diameter of the aligned masked aperture and approximately equal to the electron beam diameter which impinges the central portion of the screen. The phosphor window diameter for the perimeter portion of the screen is less than the aligned masked aperture. A method for providing such structure is detailed and involves introduction of a shader plate into the lighthouse to reduce the relative amount of developing radiation which impinges the central portion of the screen during the photoresist printing process which is used to establish the phosphor window areas.

United States Patent 11 1 Patel et al.

[ June 17, 1975 TRIAD COLOR SCREEN HAVING ELEMENTS LARGER THAN MASK APERTURES NEAR SCREEN CENTER AND SMALLER NEAR PERIPHERY [75} Inventors: Himanshu M. Patel, Horseheads;

George D. Dimmick, Bath, both of NY.

[73] Assignee: Westinghouse Electric Corporation,

Pittsburgh, Pa.

[22] Filed: Apr. 26, 1973 {211 Appl. No.: 354,579

[52] US. Cl 313/408; 313/472 [5!] Int. Cl. "01.1 29/07; H011 29/32 [58] Field of Search 3 l3/855, 402, 408

[56] References Cited UNlTED STATES PATENTS 2,947,899 8/l960 Kaplan 3l3/85 S X 3.590303 6/l97l Coleclough 3l3/92 B 3,790,839 2/1974 Dietch 313/92 R Primary Examiner-Robert Sega] Attorney. Agent, or Firm-W. G. Sutclifi' [57] ABSTRACT A color television tube of the shadow mask type with aligned phosphor windows is provided with an increased light output by providing the phosphor window diameter for the central portion of the phosphor screen of the tube having a diameter greater than the diameter of the aligned masked aperture and approximately equal to the electron beam diameter which impinges the central portion of the screen. The phosphor window diameter for the perimeter portion of the screen is less than the aligned masked aperture. A method for providing such structure is detailed and involves introduction of a shader plate into the lighthouse to reduce the relative amount of developing radiation which impinges the central portion of the screen during the photoresist printing process which is used to establish the phosphor window areas.

4 Claims, 5 Drawing Figures PATENTED 17 I975 1.8 90 52 7 saw 1 FIG.2

1 TRIAD COLOR SCREEN HAVING ELEMENTS LARGER THAN MASK APERTURES NEAR SCREEN CENTER AND SMALLER NEAR PERIPHERY BACKGROUND OF THE INVENTION This invention is related to a color television tube of the type which has a plurality of display elements or phosphor windows provided on the screen, and in which a light absorbing matrix coating surrounds the phosphor display elements. This matrix or light absorptive material surrounding the phosphor display elements has become the standard in the industry. The elements of such a tube are well known and include a phosphor screen which has a plurality of triad arranged phosphor windows with individual phosphor materials representing the three primary colors making up the triad, surrounded by an opaque light absorbing material which greatly improves the contrast for the phosphor windows. A shadow mask which has apertures aligned with the phosphor windows on the screen is used to insure that the electron beam which is to register with a particular primary color phosphor material actuates only that primary color phosphor window to which it is directed. The opaque matrix material sur' rounding the phosphor window can serve an additional purpose to absorb or act as a landing area for any broadened electron beam which exceeds the phosphor window.

A generally accepted practice has been to design the phosphor window so that it is smaller than the impinging electron beam diameter. This is termed a negative guardband structure and effectively utilizes the entire phosphor window area in producing light output. The perimeter portion of the electron beam which exceeds the phosphor window diameter is absorbed by the opaque surrounding matrix.

The controlling of electron beam landing is known to be more difficult in the perimeter portions of the tube screen than in the central portion of the tube screen. This is because of the longer path length and other focusing limitations. It has been the practice in providing negative guard-band type television tubes to provide a negative guard-band for the center portion of the screen as well as for the perimeter portions of the screen. This has been dictated by the complexity and difficulty of the manufacturing process.

SUMMARY OF THE INVENTION A color television tube exhibiting increased light output is provided comprising a hermetically sealed envelope, which includes a phosphor screen portion. The phosphor screen portion comprises a plurality of phosphor windows surrounded by opaque material. An electron gun means is provided within the envelope for directing an electron beam toward the phosphor screen. A shadow mask means is provided proximate the screen between the screen and the electron gun means. The shadow mask has a plurality of apertures which are aligned with the phosphor windows of the screen. The shadow mask apertures and the central portion of the shadow mask are slightly larger than the apertures through the remaining perimeter portion of the mask. In the improved structure the phosphor window diameter for the central portion of the phosphor screen is greater than the diameter of the aligned mask aperture and is approximately equal to the electron beam diameter which impinges the central portion of the screen. The phosphor window diameter for the perimeter portion of the screen is less than the aligned mask aperture.

A method of manufacture is detailed which permits and makes possible the above-described tube structure. It follows the basic negative guard-band photoresist process for printing opaquely surrounded windows on the screen area, but is modified by the inclusion of a shader plate of carefully controlled transmissivity disposed in the lighthouse between the light source and the screen.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the invention. reference may be had to the exemplary embodiment shown in the accompanying drawings.

FIG. 1 is a side elevational view partly in section of a color television tube of the present invention;

FIG. 2 is an enlarged partial sectional view showing the relationship of the shadow mask to the display screen;

FIG. 3 is a partial schematic representation ofa lighthouse apparatus used in practicing the method of the present invention;

FIG. 4 is an enlarged representation of a central portion of the tube screen; and

FIG. 5 is an enlarged representation of a perimeter portion of the tube screen.

DESCRIPTION OF THE PREFERRED EMBODIMENT A color television tube which is exemplary of the present invention is seen in FIG. 1. The color television tube or cathode ray tube comprises a hermetic envelope 10. The envelope 10 comprises a neck portion 12, a flared portion 14 and a faceplate panel or screen portion 16. Electron gun means l7, l8 and 19 are positioned within the envelope and during operation are respectively modulated with primary color video information. An image display screen 20 is provided on the interior surface 22 of the faceplate panel 16. The shadow mask 24 is disposed proximate the display screen but spaced therefrom between the electron gun means and the phosphor screen. The shadow mask and phosphor screen are seen in greater detail in the partially enlarged view shown in FIG. 2. A plurality of apertures 26 are provided through the shadow mask and each individual aperture 26 is aligned with a specific triad phosphor display element or windows 28. The phosphor windows 28 are arranged in triad arrangement such that there is a blue phosphor material window 288, red phosphor window 28R, and green phosphor window 28G. The electron beam path is generally designated at FIGS. 1 and 2 by the dotted lines. An opaque light absorbing material 30 is provided surrounding each of the phosphor windows 28. A thin conductive coating 32 such as aluminum is typically provided over the entire screen structure 20.

FIG. 3 is a schematic representation of a typical lighthouse apparatus of the type used in practicing the present manufacturing process for printing phosphor dots on the screen of a cathode ray tube. The faceplate l6 and shadow mask 24 seen in FIG. 3 are shown as planar elements for simplicity, whereas in actuality they are curved members. The basic lighthouse apparatus 34 comprises a substantially light tight enclosure 36 indicated by the dotted line structure. A source of radiation 38 which is typically and preferably an ultraviolet light generating source such as a high pressure mercury lamp. A light concentrator 42 is provided to produce a very small diameter point source used in the photoprinting process. The concentrator 42 tapers to a point 44 which extends through a light shield 41. A shader plate 46 of carefully controlled transmissivity is disposed between the light source and the screen. A plano-concave lens 48 is provided between the shader plate 46 in the screen and is used to focus the light image as is well known in the art. The basic process by which the phosphor screen is laid down is more fully described in US. Pat. No. 3,712,8l5 issued Jan. 23, 1973 and owned by the assignee of the present invention.

A more detailed process for producing a phosphor screen wherein the phosphor windows are smaller in area than the exciting electron beam is taught in pending Application Ser. No. 230,876, filed Mar. l, l972, entitled Method of Producing a Graphic Image," and owned by the assignee of the present invention. This teaching of the pending application is incorporated by reference into the present application. The basic teaching is that in order to print down" a phosphor window of an area smaller than the photoexposure mask aperture it is necessary that a small area light source be used, providing a relatively low light flux, and that a polar chromate ion migration enhancing additive be provided in the polymer coating which is being exposed. The light source is to be less than about 150 mils, and the radiation flux level is about of prior art levels. The polar additive is preferably N-methyl-Z- pyrrolidone or 2-pyrrolidone. The present invention is an improvement of the above described process in which in a single processing exposure phosphor windows which exceed the mask aperture are provided in the central screen area, while the phosphor windows for the perimeter portion are smaller in area than the mask aperture.

ln the present invention it is desired to produce a screen structure in which the relationship of the phosphor window diameter to the aligned aperture mask diameter is varied over the area of the phosphor screen. In order to achieve this the shader plate 46 is provided with a carefully controlled transmission reducing coating 50 such as a thin chromium metal coating which for the central portion, i.e., about two inch diameter area, of the shader plate is from 50 to 60% transmissive, i.e., the transmissivity ratio of center to edge is about 0.5 to 0.6. The transmissivity of the shader plate thereafter linearly increased to about 95% transmissivity at the edge portion of the shader plate. Thus, during the exposure process in which the phosphor windows are laid down upon the screen, the light ratio of light incident on the faceplate from the center to the edge is about 1 to 0.75. This represents an increased light intensity for the central portion of the screen over what was used in the above-described process of the copending application. The effect of increasing the transmissivity of the central portion of the shader plate is to produce a phosphor window on the screen which exceeds the mask aperture in the central portion of the screen. The increase in the light flux striking the central screen area increases the photo-reaction by which the polymeric layer is rendered insoluble during the lighthouse exposure which forms the basicinatrix. Thus, the diameter of the window'is accurately controlled.

As an example, for a mask aperture of 14.5 mils for the central portion of the shadow mask, the phosphor window diameter is about 15-16 mils, which exceeds the mask aperture by about 10%. it is appreciated that the electron beam diverges after passing through the mask aperture and will broaden to about l6 mils when it strikes the phosphor window. The screen brightness thus will be increased in the central screen portion because the phosphor window is larger than the prior art and closely matches the electron beam diameter as seen in FIG. 4, to increase the efficiency of generation of the display.

The lower light flux of center), at the edge portion of the screen permits the phosphor window on the perimeter portions of the screen to be smaller than the shadow mask aperture. By way of example, the shadow mask aperture for perimeter portions of the mask has a diameter of about l3.2 mils, and the phosphor window diameter is about 1 1.5 mils. The electron beam diameter is about 14.5 mils. Thus, in FIG. 5 the electron beam exceeds the phosphor window by several mils and the structure is what is termed a negative guard-band tube. The perimeter portion phosphor window diameter is about l0l5% smaller than the aligned mask aperture.

The guard-band or degree of overlap of the electron beam relative to the phosphor window is essentially zero in the central area of the phosphor screen, but this is not a problem because of the accurate control of beam landing in the central portion of the screen. A negative guard-band structure is maintained on the perimeter portion of this screen wherein the electron beam diameter exceeds the phosphor window diameter.

The central screen area with the approximately zero guard-band structure comprises generally the central 10-20% of the screen area.

The method detailed provides an efficient controlled exposure technique for providing a screen in which the relationship of phosphor window diameter to mask aperture is varied across the face of the screen.

We claim:

1. in a color television tube comprising; a hermetically sealed envelope, a phosphor screen faceplate portion having a plurality of triad arrays of phosphor windows with the individual phosphor windows comprising an electron beam excitable primary color emissive phosphor material surrounded by opaque material, electron gun means for directing an electron beam toward the phosphor screen, shadow mask means proximate the screen between the screen and gun means, said shadow mask having a plurality of apertures with individual apertures aligned to permit electron beam excitation of a single triad array of phosphor windows of the screen, the shadow mask apertures in the central portion of the shadow mask being slightly larger than the apertures through the remaining perimeter portion of the mask, the improvement wherein the phosphor window area for the central portion of the phosphor screen is greater than the area of the aligned mask aperture and approximately equal to the electron beam area which impinges the central portion of the screen, while the phosphor window area for the perimeter portion of the screen is less than the aligned mask aperture and electron beam area.

6 screen is smaller than the aligned mask aperture by about lO-l5%.

4. The device specified in claim 1, wherein the central screen portion comprises about 10-20% of the total screen area.

l i It t 

1. In a color television tube comprising; a hermetically sealed envelope, a phosphor screen faceplate portion having a plurality of triad arrays of phosphor windows with the individual phosphor windows comprising an electron beam excitable primary color emissive phosphor material surrounded by opaque material, electron gun means for directing an electron beam toward the phosphor screen, shadow mask means proximate the screen between the screen and gun means, said shadow mask having a plurality of apertures with individual apertures aligned to permit electron beam excitation of a single triad array of phosphor windows of the screen, the shadow mask apertures in the central portion of the shadow mask being slightly larger than the apertures through the remaining perimeter portion of the mask, the improvement wherein the phosphor window area for the central portion of the phosphor screen is greater than the area of the aligned mask aperture and approximately equal to the electron beam area which impinges the central portion of the screen, while the phosphor window area for the perimeter portion of the screen is less than the aligned mask aperture and electron beam area.
 2. The device specified in claim 1, wherein the phosphor window area for the central portion of the screen exceeds the area of the aligned mask aperture by about 10%.
 3. The device specified in claim 2, wherein the phosphor window area for the perimeter portion of the screen is smaller than the aligned mask aperture by about 10-15%.
 4. The device specified in claim 1, wherein the central screen portion comprises about 10-20% of the total screen area. 